THE SENSE OF SIGHT. 613 



In Fig. 289 let A B represent an object. From A homocentric rays 

 fall on the single refracting surface. One of the rays, the nodal 

 ray, falling on the surface perpendicularly, passes unrefracted through 

 the single nodal point, N, to the posterior focal plane. The remain- 

 ing rays, partially represented in the figure, falling on this surface 

 under varying degrees of incidence, undergo corresponding degrees of 

 refraction, by which they 

 form a converging cone 

 of rays which unite at a 

 point situated on the 

 nodal ray. These two 

 points, A, a, are known 

 as conjugate foci. The 

 same holds true for ho- 

 mocentric rays emanat- FlG ' 28 9 THE ** IMAGE IN THE 



ing from B or any other 

 point of the object. 



The Size of the Retinal Image. The size of the retinal image, 

 /, may now be easily calculated, when the size of the object, O, and 

 its distance, D, from the refracting surface with radius of curvature 

 Rj are known, by the following formula : 



O:I = D + R:F 2 R. 



For, as the triangles A N B and a N b are equal, we have 



A B : a b = / N : N g, or a b = B * N 8 . 



Independent of the foregoing method, the size of the retinal 

 image may be calculated if it is remembered that, the eye, like any 

 optic system, has a point of such a quality that a ray of light which 

 before entering the eye was directed toward it, after refraction con- 

 tinues as if it came from this point. In other words, there is in the 

 eye a point which allows a ray of light to pass unrefracted. This 

 point, termed the nodal point of the eye, determines the size of the 

 image ; for if a line be drawn from both the upper and lower ends of 

 an object through this nodal point, it is clear that the images of the 

 respective points must lie on these two rays where they intersect the 

 retina. The distance of this nodal point from the retina is 15.498 

 mm. It is clear, therefore, that the size of the object is to the size 

 of the image, as the distance of the object from the nodal point is 

 to the distance of the nodal point from the retina. 



The Visual Angle. The_angle included between the lines j:oming 

 from the opposite extremities oT~an object and ^crossmg^ at the 

 nodal point is termed tne visual angle. The size of this angle in- 

 creases with the nearness "anoT^ecreases with fEe remoteness of an 

 object. The retinal image correspondingly increases or decreases 

 in size. The acuteness of vision depends on the power of the emme- 

 tropic eye to distinguish the smallest retinal image or the smallest 



